1. Patient size AEC. Large Patient High ma. Small Patient Low ma

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1 Comparison of the function and performance of CT AEC systems CTUG meeting by Emily Field Trainee clinical scientist 14 th th <Date>

2 Breakdown CT Automatic Exposure Control (AEC) Background Project Description Aim Methodology Results Conclusion 14 th October <Date> 21

3 AEC systems in CT What is the aim of AEC in CT? To minimise or remove variations in image quality between different images. To reduce variation in doses delivered to patients of varying sizes/shapes. How is this achieved? This is made possible in CT scanning by controlling the tube current (ma) during scanning to achieve the required level of image noise. This is also known as ma modulation. Modern CT scanners can achieve ma modulation in 3 distinct ways

4 AEC systems in CT 1. Patient size AEC ma is adjusted grossly based upon the overall size of the patient. Large Patient High ma Small Patient Low ma

5 AEC systems in CT 2. Z-axis AEC Variations in attenuation along the length of the patient are compensated for by adjusting the ma for each successive tube rotation. tube current ma z-axis position Pelvis High ma Neck Low ma

6 AEC systems in CT 3. Rotational AEC ma is adjusted during a single rotation of the tube to compensate for differences in attenuation between AP and Lateral projections. AP Projection Low ma Lateral Projection High ma

7 AEC systems in CT In reality, all three AEC systems operate simultaneously. Patient and z-axis AEC The main source of patient attenuation data necessary for operation of the AEC system is acquired during the scan projection radiographs (SPRs). These are also known as scouts, topograms or scanograms. Rotational (x, y axis) AEC Feedback - changes in patient profile occur gradually along the z-axis, many systems utilise real-time feedback to inform the system of the changes in attenuation. For example, the patient attenuation data acquired during a single rotation can be used to inform the system of the optimum ma settings for the subsequent rotation. SPR - asymmetry of the patient can be estimated from SPRs and the x- ray tube current varied accordingly.

8 AEC systems in CT - Benefits What are the overall benefits? Consistent image quality - User defined levels of image noise achievable from slice to slice but also from patient to patient. Potential to reduce patient exposure - A fully optimised CT system can avoid unnecessary exposure of the patient. Reduced tube loading - Modulated ma runs have the potential to reduce the overall loading of the x-ray tube. Extended scan runs - A reduction in x-ray tube heating means that longer scan runs can be utilised where necessary. Reduction of photon starvation artefacts - Rotational AEC means that previously under-sampled lateral projections (e.g. across shoulders) can be avoided.

9 AEC systems in CT Each major CT manufacturer has their own version of AEC. Manufacturer Patient size AEC Z-axis AEC Rotational AEC Method for setting exposure level GE AutomA AutomA SmartmA Noise Index Siemens Care Dose 4D Care Dose 4D Care Dose 4D Reference mas Toshiba Sure Exposure Sure Exposure Sure Exposure 3D Standard deviation Philips DoseRight ACS - DoseRight DOM Reference image noise level

10 Study Aim To assess the efficacy of a range of CT scanner AEC systems using a homogeneous elliptical cone phantom. Variations in performance characteristics between scanner models and manufacturers was also investigated.

11 Scanners Tested Hospital Site A B C D E F G H Scanner CT1 CT2 CT CT CT CT1 CT2 CT3 CT CT CT Make/Model Slice Toshiba Aquilion 64 Toshiba Aquilion 16 Toshiba Aquilion 64 Toshiba Aquilion 64 Toshiba Aquilion 64 GE Discovery HD75 (dual kv) with ASIR 64 GE Lightspeed 16 GE Lightspeed 64 GE Lightspeed with (ASIR) GE Lightspeed 64 Siemens Sensation 64 8

12 Methodology The Phantom Homogeneous, acrylic, elliptical, cone- shaped phantom. Same phantom used by ImPACT for their 25 report 1 (Thank you!). Designed to test each distinct AEC system (z-axis, rotational etc ) Phantom dimensions (3cm z-axis length). 1 CT scanner automatic exposure control systems. Medicines and Healthcare Regulatory Agency, February 25. Report 516

13 Methodology Standard Settings Standardised test protocol for every CT scanner attempted for fair comparison (12kVp, 1sec rot time, standard reconstruction parameters, 5mm slice recon, large FOV). However, slight variations unavoidable between models/manufacturers (below). Manufacturer Detector Rows Collimation (mm) GE 64 2x5 Helical /Axial Pitch Axial x1.25 Axial - AEC system AutomA, SmartmA AutomA, SmartmA 8 8x1.25 Axial - Auto ma Siemens 64 64x.6 Helical.6 CARE Dose 4D Toshiba 64 16x.5 Helical.938 SureExposure4D 16 16x.5 Helical.938 SureExposure4D Image quality settings NI 1, 1-75mA NI 1, 1-75mA NI 1, 1-44mA Average, 21 quality ref SD 1, 1-38mA SD 7.5, 8-41mA

14 Methodology Scan projection radiographs (SPRs) acquired (AP and lateral) along entire phantom length. Scans of complete phantom length planned and performed from SPR images (based on previously described standard settings). Resulting sequence of images analysed in terms of two key parameters; 1. Delivered ma for each slice (related to absorbed dose) 2. Standard deviation of CT numbers in central ROI (measure of noise) The effect of adjusting several parameters on applied x-ray tube current and image standard deviation (noise level) were recorded; kv Pitch Reconstruction kernel AEC image quality setting e.g. noise index 145mA 159mA 17mA 172mA 18mA SD=xx SD=xx SD=xx SD=xx SD=11.7

15 Results The following results are for 64-slice scanners only

16 AEC on/off

17 Results - GE SD of CT num ber SmartmA, AutomA 1 AutomA Fixed ma (2mA) X-ray tube c urrent (m A) SmartmA, AutomA 1 AutomA Fixed ma (2mA) AEC system maintains image quality as the AP diameter is increased. Image quality is improved when AutomA is combined with SmartmA 14 th October <Date> 21

18 Results - Toshiba SD of CT number x,y and z-axis z-axis fixed ma (2mA) X -ray tube c urrent (m A) x,y and z-axis z-axis Fixed ma (2mA) AEC system maintains image quality as the AP diameter is increased. Image quality is improved when z-axis AEC is combined with x, y- axis AEC.

19 Results - Siemens Siemens z-axis and x, y-axis AEC system CareDose4D could not be operated independently. CareDose4D could either be selected with both AEC systems working together or not at all.

20 Varying image quality

21 Results - GE SD of CT number mA X-ray tube current (ma) mA Increasing image quality increases ma. Image quality is maintained at the required level more accurately at lower NI values.

22 Results - Siemens 25 CareDose4D (21mAs) 35 CareDose4D (21mAs) S D of C T num ber CareDose4D (5 mas) Fixed mas (333mAs) X-ray tube current (ma) CareDose4D (5mAs) Fixed mas (333mAs) Increasing the quality reference mas increases the ma modulation. Image noise reduced for smaller AP diameters than larger ones. Therefore reducing the image noise for smaller patients and increasing it for larger ones (where it is more tolerable).

23 Results - Toshiba SD of CT number Fixed ma (2mA) X-ray tube current (ma) Fixed ma (2mA) AEC system behaves in a similar fashion to GE Increasing image quality increases ma. Image quality is maintained at the required level more accurately at lower SD values. Increasing SD increases the AP diameter at which ma begins to be modulated.

24 Varying tube voltage

25 Results - GE SD of CT number kv 14kV X-ray tube current (ma) kV 14kV Changing kv does not alter image quality. ma decreases when kv increases to maintain image quality.

26 Results - Siemens SD of CT number kv 12 kv X-ray tube current (ma) kV 12 kv Increasing kv increases the image quality, whilst ma remains constant at AP diameters greater than 13mm.

27 Results - Toshiba SD of CT number kv 12 kv X-ray tube current (ma) kV 12 kv Changing kv does not alter image quality. ma decreases when kv increases to maintain image quality th th October <Date> 21

28 Varying reconstruction kernel

29 Results - GE SD of CT num ber Std Soft Edge X-ray tube current (m A) Std Soft Edge Changing the reconstruction algorithm changes the variation in CT number for each pixel making up the image to alter the appearance. Reconstruction algorithm chosen did not alter the ma applied.

30 Results - Siemens 7 6 B5s medium sharp B3s medium smooth B5s medium sharp B3s medium smooth SD of CT number X-ray tube current (ma) Behaves similar to GE. Changing the reconstruction algorithm changes the variation in CT number for each pixel making up the image to alter the appearance. Reconstruction algorithm chosen did not alter the ma applied.

31 Results - Toshiba SD of CT number Std large body Std soft tissue Std lung X-ray tube current (ma) Std large body Std soft tissue Std lung Changing the reconstruction algorithm changes the variation in CT number for each pixel making up the image to alter the appearance. Reconstruction algorithm chosen alters the ma applied along the phantom length.

32 Varying pitch

33 Results - GE SD of CT number X-ray tube current (ma) AP Diameter (mm) AP Diameter (mm) Scan mode changed to helical with 4mm collimation to produce 5mm slices. Lowering pitch causes ma along the phantom to be reduced in order to achieve the NI specified. Reduced image noise between 1 and 25mm for increased pitch.

34 Results - Siemens SD of CT number X-ray tube current (ma) Changing pitch has a similar effect on GE, Siemens and Toshiba 64 slice scanners. Lowering pitch causes ma along the phantom to be reduced in order to achieve the image quality specified.

35 Results - Toshiba SD of CT number X-ray tube current (ma) Changing pitch has a similar effect on GE, Siemens and Toshiba 64 slice scanners. Lowering pitch causes ma along the phantom to be reduced in order to achieve the image quality specified.

36 Intra-manufacturer variation

37 Results - GE SD of CT number slice dual kv with ASIR 64 slice 64 slice with ASIR X-ray tube current (ma) slice dual kv with ASIR 64 slice 64 slice with ASIR Scanners with ASIR capabilities maintain image quality selected greater than the scanner without. The same ma modulation is apparent for the three scanners.

38 Results - Toshiba SD of CT number CT1 A B C D X-ray tube current (ma) CT1 A B C D Comparison of AEC system performance of four 64 slice Toshiba Aquilions. CT1 A and B behave differently to C and D with the same settings applied. C and D maintain greater image quality at increased AP diameters.

39 Beware older scanners & software!

40 Results - Toshiba SD of CT number mA X-ray tube current (ma) mA Performance of 16 slice Toshiba was found to be different in respect to the Toshiba 64 slice scanners. Older software version, different AEC system interface. ma modulation only occurs over a small AP diameter range (~5cm). Point at which ma modulation occurs increases with AP diameter.

41 Conclusion Scanners tested performed consistently with the findings described in the 25 report by ImPACT 1. Machines with apparently identical operation were found to have AEC systems which performed differently. Only through the individual testing of each scanner can the true behaviour of its AEC system be established. It is therefore essential that users operating each scanner fully understand not only how the relevant manufacturers AEC systems work in general but also how the specific scanner in their department operates. 1 CT scanner automatic exposure control systems. Medicines and Healthcare Regulatory Agency, February 25. Report th

42 Thank You